The Role of Along‐Fault Dilatancy in Fault Slip Behavior.

Earthquakes result from fast slip that occurs along a fault surface. Interestingly, numerous dense geodetic observations over the last two decades indicate that such dynamic slip may start by a gradual unlocking of the fault surface and related progressive slip acceleration. This first slow stage is of great interest, because it could define an early indicator of a devastating earthquake. However, not all slow slip turns into fast slip, and sometimes it may simply stop. In this study, we use a numerical model based on the discrete element method to simulate crustal strike‐slip faults of 50 km length that generate a wide variety of slip‐modes, from stable‐slip, to slow earthquakes, to fast earthquakes, all of which show similar characteristics to natural cases. The main goal of this work is to understand the conditions that allow slow events to turn into earthquakes, in contrast to those that cause slow earthquakes to stop. Our results suggest that fault surface geometry and related dilation/contraction patterns along strike play a key role. Slow earthquakes that initiate in large dilated regions bounded by neutral or low contracted domains, might turn into earthquakes. Slow events occurring in regions dominated by closely spaced, alternating, small magnitude dilational and contractional zones tend not to accelerate and may simply stop as isolated slow earthquakes. Plain Language Summary: Numerous research studies indicate that earthquakes may start progressively by a slow unlocking of the fault surface followed by fast slip acceleration. This first slow‐slip stage could then define an early indicator of a devastating earthquake. However, not all slow‐slip become fast, and some may simply stop. In this study, we use a numerical model to simulate faults that generate both slow earthquakes and fast earthquakes, sharing similar characteristics to natural cases. The main goal is to understand the conditions that allow slow events to turn into earthquakes, in contrast to those that cause slip to abort. Our results suggest that fault surface geometry and induced strain patterns play a key role. Slow events that initiate in large dilated regions might turn into earthquakes whereas the ones occurring in regions showing more complex strain pattern including contracted zones may simply stop as isolated inoffensive slow earthquakes. This work could have significant implications for the development of new monitoring systems devoted to anticipate the occurrence of destructive earthquakes. Key Points: A discrete element model is used to investigate earthquake nucleation mechanismsFault geometry and precursory dilation/contraction patterns along strike might be predictive by controlling the earthquake energy balanceSlow events in dilated zones bounded by neutral areas turn into earthquakes whereas those in alternating dilated/contracted zones stop [ABSTRACT FROM AUTHOR]